Abstract

The maximum shear modulus and liquefaction resistance of soils are fundamental input parameters in the non-linear dynamic analyses of soil structures. This paper focuses on the dynamic behaviour of natural pumiceous soils, which originated from a series of volcanic eruptions centred in the Taupo and Rotorua regions and then mixed with other materials during re-working and then re-deposition. They are highly crushable, compressible and lightweight due to the vesicular nature and presence of internal voids in the pumice particles, making them problematic from an engineering point of view. The test specimens were obtained from a site near Rangiriri, Waikato using a block sampling method. Cyclic undrained triaxial tests were conducted on specimens using triaxial apparatus equipped with bender elements. The maximum shear modulus of the pumiceous materials was assessed by considering the effect of confining pressure. Moreover, the liquefaction resistance curves from cyclic loading tests were measured and compared with those of reconstituted hard-grained Toyoura sand. In addition, to distinguish the crushable volcanic soils from the hard-grained sands, image analyses on both pumice and Toyoura sand particles using scanning electron microscopy were performed.

Introduction

It is well-known that earthquakes can have disastrous effects on infrastructure, buildings and society. One reason for earthquake damage is related to soil failure under seismic loading. For instance, widespread liquefaction was one of the main causes of damage following the 2010-2011 Canterbury Earthquake Sequence in New Zealand (Cubrinovski et al. 2010; 2011). Consequently, understanding the geotechnical characteristics and seismic behaviour of various local soils is important for the purpose of designing earthquake resistant structures.

Volcanic soils, including pumiceous sands, are found in several areas of the North Island. They originated from a series of volcanic eruptions centred in the Taupo and Rotorua region, called the Taupo Volcanic Zone (Pender et al. 2006). By the power of explosion and airborne transport which were followed by erosion and river transport, the pumice materials were distributed and mixed with other materials in the Waikato Basin. As a consequence of infrastructure development in the region, many engineering projects frequently encounter pumiceous materials, so there is a need to understand how these deposits behave under seismic loading. Pumice is characterised by a number of distinctive properties. Pumice particles are highly crushable, compressible and lightweight as a result of the vesicular nature and the presence of internal voids (Orense et al. 2012). Due to these features, it is found that pumice deposits are problematic from an engineering point of view. Another important characteristic of pumice is its unique appearance (Figure 1) with the particles having high angularity that enables them to have very high angles of internal friction (Asadi et al. 2015; Asadi et al. 2016; Kikkawa et al. 2013). Although some studies have investigated the dynamic properties and cyclic behaviour of crushable soils such as carbonate sand and commercially-available pumice, little is known about the seismic response of natural pumiceous deposits which are a mixture of pumice with other constituents.

Material used and test programme

Site location and sampling methodology

The samples used were undisturbed pumiceous deposits from a site near Rangiriri, Waikato (see Figure 2). In order to get reasonable results from laboratory tests, it is necessary to have high-quality undisturbed samples because soil fabric, age and stress history play an important role in the dynamic behaviour of soils (Ishihara 1993). Thus, for this study, a small block sampling method was implemented to obtain undisturbed pumiceous soil samples. After excavating the ground to reach the target depth of 4.0 – 4.5m, a hydraulic jack was used to push 200 mm diameter and 200 mm high sampling tubes which were fitted with low angle cutting shoe into the ground. The tubes were then dug out of the ground, and both ends were levelled and sealed with rubber discs between the soil and caps to preserve the natural water content of soil samples.

Specimen preparation and triaxial test set up

After obtaining the undisturbed block samples, the materials were transported to the laboratory in Auckland. In the laboratory, a hydraulic jack was used to extrude the undisturbed soil samples from the sampling tube. Then, this was followed by cutting them into four pieces using a bandsaw, and subsequent trimming using a soil lathe to obtain specimens with target size of 126 mm high and 63 mm diameter. Moreover, for the purpose of comparison, another set of tests were performed on Toyoura sand which is known as a hard-grained, sub-angular material and commonly used in laboratory tests in Japan. The reconstituted Toyoura sand specimens for the triaxial tests were prepared by the moist tamping method, with the target specimen size of 63mm diameter and 126mm high. Prior to the sample preparation, the soil materials were mixed with water (approximately 20% of soil weight) to form uniformly moist samples. Then the specimens were prepared in a split mould with the membrane in place to achieve different target initial relative density. After the samples reached the target height, the top cap was positioned and the membrane sealed with O-rings, then the split mould was removed. Next, the specimens were saturated by subjecting them to back pressure of 600 kPa. B-values greater than 0.95 ensured that all the specimens were fully saturated. Then, the specimens were isotropically consolidated at the target effective confining pressures. The index properties of the materials tested are illustrated in Table 1.

The Japanese standard method (JGS 2000) was adopted to measure the index properties of the materials. Note that the pumiceous soils have a lower Gs and larger void ratio range than Toyoura sand.

Bender element test

The bender element test is a non-destructive dynamic method developed by Shirley and Hampton (1978). This test enables the measurement of shear wave velocity, Vs, of soil in laboratory by the following equation:

Vs = L/t

Equation 1

where L is distance between tip-to-tip of bender elements and t is the travel time of wave propagation. Then, based on theory of wave propagation in elastic media, the maximum shear modulus of soil is measured by Equation (2):

Gmax = ρVs2

Equation 2

where ρ is the bulk density of soil. The bender element system employed consisted of a FG110 synthesized function generator and a TDS 2024C digital oscilloscope for generating waves and recording the generator and receiver signals for post data analysis, respectively. In bender element data analysis, the peak-to-peak arrival time was chosen to determine travel time, owing to the consistency, clarity and simplicity of this method as compared to other approaches.

Cyclic triaxial tests

After consolidating the specimens with an effective confining pressure of 100 kPa, a servo-hydraulic loading frame applied the cyclic loading during the tests. All the specimens were subjected to a sinusoidal cyclic axial load with a frequency of 0.1 Hz under undrained condition. After the double amplitude axial strain reached 5%, the cyclic loading application was stopped. Furthermore, the axial load, displacement, cell pressure and back pressure were electronically recorded through a data acquisition system with a 16 bit A/D conversion into a computer for analysis.

Particle shape index

In order to distinguish the pumiceous sands from the other types of hard-grained soil particles, 2D scanning electron microscope (SEM) images were taken on different particle sizes of pumice soils and Toyoura sand at their most stable configuration. Subsequently, the methodology of Kikkawa et al. (2013) was followed to analyse the SEM images and quantify the soil particle shape characteristics through roundness coefficient (Rc), aspect ratio (Ar) and angular coefficient (Ac), which are defined as follows:

Rc=L2/4πA

Equation 3

Ar=b/a

Equation 4

Ac=|Rc – 1 + Ar2/(2Ar)|

Equation 5

In the above equations, L is the perimeter, A is the surface area, while a and b are the dimensions of the particles along the minor and major axes, respectively. Each of the aforementioned parameters (Rc, Ar and Ac) indicates an important feature of a soil particle. For instance, if the value of Rc is equal to one, the particle is circular and if it exceeds unity, the shape would change to ellipsoidal. Furthermore, if Ar > 1, the soil particle is more elongated. A higher value of Ac illustrates a more angular particle surface. While the interaction between particles within the triaxial specimen is essentially 3D, these 2D indices can very well characterize the shape of the particles. The average image analysing results are summarized in Table 2 and it is evident that the particle shape indices of natural pumiceous sand are considerably different from those of Toyoura sand. For example, the value of Rc for natural pumiceous soils is approximately 1.5 times higher than that of Toyoura sand. Furthermore, the average Ar value for natural pumice soil is 1.812, compared to 1.483 for Toyoura sand. The Ac value of the natural pumiceous soils is about 5 times higher than that of Toyoura sand.

Table 2: Particle shape indices of pumice sand and Toyoura sand

Soil Type

Number of analysed soil particle images

Average Rc

Average Ar

Average Ac

Natural pumice soil

98

1.703

1.812

0.521

Toyoura sand

50

1.258

1.483

0.179

Test results

It has long been established that the maximum shear modulus (Gmax) of sandy soils is significantly influenced by effective confining pressure (σ’c) and void ratio (e) (Kokusho 1980). In order to investigate the effect of confining pressure on the Gmax of undisturbed natural pumiceous soils, several series of bender element tests at different levels of σ’c(ranging from 50 to 600 kPa) were performed on a single specimen. Furthermore, for the purpose of comparison similar tests conducted on the reconstituted hard-grained Toyoura sand and some results after

Figure 3 presents the Gmax – σ’crelationship for undisturbed pumiceous material and compared with Toyoura sand, Silica sand and Pumice-A sand. As evident from the graph, the Gmax of natural pumiceous soil is considerably lower than that of Toyoura sand, Silica sand and Pumice-A sand at all levels of σ’c. In addition, Figure 3b illustrates that the dependence of Gmax for natural pumiceous soil on σ’cis significantly more pronounced when compared to other soils. For instance, in the well-known Gmax ∝ σ’cm relationship, the m value, representing the slope of the best-fit line through the data points, is 0.88 for undisturbed natural pumiceous soil and almost similar to that of Pumice-A sand with m=0.80. However, the m value for hard-grained Toyoura sand and silica sand are significantly lower, with values of 0.54 and 0.53, respectively.

The substantial differences in Gmax as well as in Gmax– σ′crelation for natural pumiceous soil and hard-grained soils (i.e. Toyoura and Silica sands) can be explained by soil particle morphology and the crushability feature of pumice particles. Therefore, the lower value of Gmax for natural pumiceous soil compared to hard-grained materials could be owing to the distinctive pumice particle characteristics such as porosity, lower unit weight and brittle feature (Wesley 2001; Orense et al. 2012). Furthermore, Figure 4 indicates that the shear wave velocity (Vs) of natural pumiceous soil is lower than that of Toyoura sand. It is well-known that the Vs propagation in soft soil deposits is considerably lower than in stiff soils and, consequently, using a lower value of Vs and unit weight for natural pumiceous soil in Equation (2) would result in lower Gmax for pumiceous soil when compared to Toyoura sand.

On the other hand, the higher Gmax–σ’cdependence of pumiceous soils compared to hard-grained sand can be explained by a better contact area between pumice particles, higher angularity and manifestation of particle crushing. As the isotropic consolidation test results on the tested materials shown in Figure 5 indicate, the natural pumiceous soil illustrates higher decrease in void ratio with respect to the increase in σ’c during the test compared to Toyoura sand. In order to indicate the changes in void ratio during consolidation test more clearly, the void ratios at all levels of σ’c were normalized by the initial void ratio (einitial) in Figure 5b. The plots indicate that the increment in contact area between pumice particles is more than that of Toyoura sand at each level of consolidation pressure, resulting in a higher m value. Moreover, Cho et al. (2006) investigated that as the soil particles tend to be more angular and elongated, the m value would increase. Along this line, the higher angularity and elongation of natural pumiceous soil, which are shown in Table 2, could be an additional reason for higher m value for natural pumiceous soil compared to Toyoura sand. In addition, the higher angularity and elongation of pumice particles would help the soil assembly to have better interaction (interlocking) with each other and, as a result, have better contact area as σć increases, leading to a higher rate of increase in Gmax than occurs for hard-grained sand.

Liquefaction resistance

Figure 6a compared the liquefaction resistance of undisturbed natural pumiceous deposits (Dr=50%) with commercially-available pumice sands. It is noted that the undisturbed pumiceous deposits are more resistant to liquefaction compared to reconstituted commercially-available pumice. This can be explained partially by the contribution of soil fabric and stress history on the liquefaction resistance of the soil, although the effect of other factors (such as difference in fines content and composition) may also be significant.

From Figure 6b, it can be seen that the pumiceous materials are less susceptible to liquefaction compared to Toyoura sand. For example, if the cyclic resistance is defined in terms of the cyclic stress ratio (CSR) corresponding to 15 cycles, then dense Toyoura sand (Dr=90%) has liquefaction resistance almost half of that of undisturbed pumiceous materials (Dr=50%). The higher liquefaction resistance of pumiceous soils compared to Toyoura sand is partly due to the consequences of particle crushing during the cyclic test and rearrangement of the soil skeleton resulting in a more stable soil structure (Asadi et al. 2017). More tests are planned to confirm this phenomenon. It is worth mentioning that Yamawaki et al. (2002) noted that as the values of Rcand Arof various geomaterials increase, the liquefaction resistance would also increase. In the same vein, the higher angularity of pumiceous soils may be a reason for their high liquefaction resistance when compared to Toyoura sand.

Conclusions

In order to investigate the dynamic properties of pumiceous soils from the Waikato basin in the North Island, New Zealand, several series of bender element tests and undrained cyclic triaxial tests were performed on undisturbed pumiceous soils. Similar tests were also conducted on reconstituted hard-grained Toyoura sand. Moreover, to distinguish the pumice particles from hard-grained sand, SEM images of the soil particles were analysed to illustrate the differences. The following are the major conclusions of this study:

The effect of confining pressure on the maximum shear modulus of natural pumiceous soil was more significant compared to non-crushable soil.

The liquefaction resistance of undisturbed natural pumiceous materials (Dr=50%) was found to be 1.6 times higher than that of medium dense pumice sand (Dr=70%) and almost twice that of dense Toyoura sand (Dr=90%).